It is known in the art of hydrocarbon recovery, and particularly in the recovery of heavy hydrocarbons from subsurface reservoirs, to employ the use of steam or steam-solvent mixtures as an injectant to reduce the viscosity of hydrocarbon housed in a reservoir thus allowing the hydrocarbon to flow to a producing well and thereby be produced to surface. Injected fluids can thermally interact with the hydrocarbon in a manner that allows the hydrocarbon to mobilize and then be produced.
For example, cyclic steam stimulation (CSS) and steam-assisted gravity drainage (SAGD) methods employ steam to mobilize subsurface hydrocarbon such as heavy oil or bitumen.
CSS requires a predetermined amount of steam to be injected into a well drilled into the hydrocarbon deposit, which well is then shut in to allow the steam and heat to soak into the reservoir surrounding the well. This assists the natural reservoir energy by thinning the oil (or, in the case of a steam-solvent injection, also mixing the heavy hydrocarbon with lighter hydrocarbons) so that it will more easily move into the production well. Once the reservoir has been adequately heated, the well can be put into production until the injected heat has been mostly dissipated within the fluids being produced and the surrounding reservoir rock and fluids. This cycle can then be repeated until the natural reservoir pressure has declined to a point that production is uneconomic, or until increased water production occurs.
SAGD involves a pair of horizontal wells that are drilled into a hydrocarbon reservoir. The upper wellbore is typically referred to as the injector well, while the lower wellbore may be referred to as the producer well. In a SAGD hydrocarbon recovery operation, high pressure steam is continuously injected into the upper wellbore to heat the hydrocarbon and reduce its viscosity. The heated hydrocarbon drains into the lower wellbore as a result of gravity. The resulting hydrocarbon in the lower producer wellbore may be pumped to surface.
There are, however, many known drawbacks to using steam during thermal hydrocarbon recovery operations. For example, it has long been recognized that such recovery methods can be costly to implement and operate and require access to significant water resources. Further, in some reservoirs such as Lloydminster-type reservoirs, which are thin heavy oil reservoirs, these conventional steam-based recovery methods generally cannot be employed due to significant heat loss to the overburden and underburden.
Some prior art solutions employ heaters that are positioned in a well which heat the hydrocarbon housed in a reservoir while reducing hydrocarbon viscosity. Heaters typically, however, have issues with reliability and can be difficult to maintain. Furthermore, the use of heaters can be costly if they need to be positioned over a large area.
U.S. Pat. No. 7,621,333 to Marchal addresses some of the drawbacks of the thermal hydrocarbon recovery methods discussed above, by producing hydrocarbon to surface, heating the produced hydrocarbon and then re-injecting the produced hydrocarbon downhole, and circulating it within the wellbore to thermally reduce hydrocarbon viscosity in the reservoir allowing the reservoir hydrocarbon and the re-injected hydrocarbon to be produced together to surface. Marchal does not contemplate, however, the problems associated with adjacent horizontal wellbores that compete for production of the same hydrocarbon resulting in wasted energy during the hydrocarbon recovery process. Furthermore, Marchal would require expensive surface separation technology to generate the hydrocarbon for re-injection.